The aim of the proposed research is to fabricate and test a small scale model of what should be the ultimate radiotherapy imager. It is called the Port Digital Imager (PDI). The PDI is technically innovative with respect to image quality, dynamic range and speed of image acquisition. It is a real-time, very low noise, large dynamic range digital system whose image quality is limited only by photon statistics, even at the low doses preferred for localization images.The PDI has the highest detective quantum efficiency (DOE) possible and will provide the highest resolution necessary without trading off resolution for sensitivity. This high performance will mean a lower patient imaging dose and more frequent localization imaging. In addition, the imager is flat and cassettelike and will be as physically convenient as film. The methodology is strongly based upon testing and verification of the Phase I calculation results. The specific aims are to verify: that the PDI DOE is limited only by photon statistics; the high acquisition speed of the device; the effectively infinite dynamic ran that the spatial resolution is in fact determined by the detector size; that the gradual total dose impact upon the PDI can be simply accounted for by adjusting computer-stored gain and linearity tables. An additional aim is to develop the fabrication and testing knowledge that will allow the PDI to be easily commercialized.